Validate required API credentials are there otherwise show error message

This prevents a nil value to propagate down to the HTTP client and then
emerge in the error page as an exception.

app/controllers/application_controller.rb

@@ -6,6 +6,7 @@
 
 class ApplicationController < ActionController::Base
 
+  before_action :validate_configuration
   before_action :set_language
   before_action :set_distributions
   before_action :set_baseproject
@@ -32,6 +33,16 @@ class MissingParameterError < Exception; end
     render :template => 'error', :formats => [:html], :layout => layout, :status => 400
   end
 
+  def validate_configuration
+    config = Rails.configuration.x
+    layout = request.xhr? ? false : "application"
+
+    if config.api_username.blank? && config.opensuse_cookie.blank?
+      @message = _("The authentication to the OBS API has not been configured correctly.")
+      render :template => 'error', :formats => [:html], :layout => layout, :status => 503
+    end
+  end
+
   def set_language
     set_gettext_locale
     requested_locale = FastGettext.locale

test/integration/validate_configuration_test.rb

@@ -0,0 +1,17 @@
+require File.expand_path('../../test_helper', __FILE__)
+
+class ValidateConfigurationTest < ActionDispatch::IntegrationTest
+  def test_validate_configuration
+    # Fake that authentication is not configured
+    x = Rails.configuration.x.clone
+    Rails.configuration.x.api_username = nil
+    Rails.configuration.x.opensuse_cookie = nil
+
+    visit '/'
+    page.assert_text 'The authentication to the OBS API has not been configured correctly.'
+    assert_equal 503, page.status_code
+
+    # Restore normal configuration values for other tests
+    Rails.configuration.x = x
+  end
+end